Optical displays are widely used for lap-top computers, hand-held calculators, digital watches and the like. The familiar liquid crystal (LC) display is a common example of such an optical display. The conventional LC display locates a liquid crystal and an electrode matrix between a pair of absorptive polarizers. In the LC display, portions of the liquid crystal have their optical state altered by the application of an electric field. This process generates the contrast necessary to display “pixels” of information in polarized light.
For this reason the traditional LC display includes a front polarizer and a rear polarizer. Typically, these polarizers use dichroic dyes which absorb light of one polarization orientation more strongly than the orthogonal polarization orientation. In general, the transmission axis of the front polarizer is “crossed” with the transmission axis of the rear polarizer. The crossing angle can vary from zero degrees to ninety degrees. The liquid crystal, the front polarizer and rear polarizer together make up an LCD assembly.
LC displays can be classified based upon the source of illumination. “Reflective” displays are illuminated by ambient light that enters the display from the “front.” Typically a brushed aluminum reflector is placed “behind” the LCD assembly. This reflective surface returns light to the LCD assembly while preserving the polarization orientation of the light incident on the reflective surface.
It is common to substitute a “backlight” assembly for the reflective brushed aluminum surface in applications where the intensity of the ambient light is insufficient for viewing. The typical backlight assembly includes an optical cavity and a lamp or other structure that generates light. Displays intended to be viewed under both ambient light and backlit conditions are called “transflective.” One problem with transflective displays is that the typical backlight is not as efficient a reflector as the traditional brushed aluminum surface. Also the backlight randomizes the polarization of the light and further reduces the amount of light available to illuminate the LC display. Consequently, the addition of the backlight to the LC display makes the display less bright when viewed under ambient light.
Therefore, there is a need for a display which can develop adequate brightness and contrast under both ambient and backlight illumination.